Audiobus: Use your music apps together.

What is Audiobus? — Audiobus is
an award-winning music app for iPhone and iPad which lets you use
your other music apps together. Chain effects on your favourite
synth, run the output of apps or Audio Units into an app like
GarageBand or Loopy, or select a different audio interface output
for each app. Route MIDI between apps — drive a
synth from a MIDI sequencer, or add an arpeggiator to your MIDI
keyboard — or sync with your external MIDI gear.
And control your entire setup from a MIDI controller.